Lightweight foamed polystyrene articles have been used in a variety of applications as heat insulation materials for buildings, houses and household electrical appliances and as materials for earth-piling in civil engineering technology.
Polystyrene is consisted of solely carbon and hydrocarbon atoms and very inflammable. Once ignited, polystyrene foams burn rapidly with evolution of black smoke. Consequently, it is imperative in some application fields to impart polystyrene foams with flame retardancy by the addition of flame retardants. Bromine-containing organic compounds are widely used for this purpose. In order to accomplish high flame retardancy with minimum amount of addition, a flame retardant in which all brome atoms are attached to aliphatic carbon atoms such as hexabromocyclo-dodecane (HBCD) is especially effective. However, such a flame retardant exhibits poor heat stability and causes problems such as deterioration and darkening of styrene-based resins, corrosion of processing machines due to generation of hydrogen bromide gas.
Certain flame retardants having high heat stability are known including tetrabromobisphenol A-bis(2,3-dibromopropyl ether). These flame retardants are lower in the flame retardant effect than HBCD and like aliphatic flame retardants and require a larger amount of addition than HBCD and like aliphatic flame retardants to accomplish the same level of flame retardancy. Therefore, the use of large amounts of flame retardants having relatively high heat stability causes problems of decreasing the physical properties of styrene-based resins and increasing the production cost of foamed articles made therefrom.
Polystyrene foams are produced by melting the starting polystyrene together with any desired additives, mixing a blowing agent into the melt and extruding the melt to the atmosphere at a low temperature from an extruding machine. When thermally unstable flame retardants such as HBCD are used to produce flame-retarded polystyrene foam by the above process, deterioration of the polymer such as depolymerization and darkening occurs necessarily. In order to alleviate this problem, it has been proposed in JP 2006316251A to use a heat stabilizing agent together with the flame retardant.
Recently, a demand for recycling of polystyrene foam arises as environmental concern enhances. In the recycled use, polystyrene foams to be recycled are subjected to heating repeatedly and hence require further improved heat stability than required heretofore.
JP 2005139356A proposes to employ tetra-bromobisphenol A-bis(2-methyallyl ether) or tetra-bromobisphenol A-bis(2,3-dibromo-2-methylpropyl ether) when producing polystyrene foams suitable for recycling. However, the polystyrene foams are not satisfactory in the heat stability for recycling.